The present invention relates to a power supplies, and in particular to high voltage power supplies.
High voltage generator circuits are used to provide power for a variety of applications that rely on the acceleration of charged particles. For example, high voltage generators are used in ion implanter systems for the manufacture of semiconductors, electron beam irradiation systems, x-ray generators, isotope production systems for medicine, research and industry, neutron production systems, accelerator mass spectrometers, research accelerators, and other applications. These applications require the use of high voltage power supplies capable of generating voltages ranging from several kilovolts to a few megavolts, and power levels of several watts to many tens of kilowatts.
High voltage generator circuits often include a high voltage multiplier-rectifier circuit, an AC drive circuit, and a transformer interface connecting the AC drive circuit to the multiplier-rectifier circuit. To achieve good performance, it is common practice to operate the AC drive circuit at frequencies ranging from several kilohertz to several hundred kilohertz. It is also common practice to utilize interface transformers with large step-up turns ratios ranging from 10:1 to 1000:1, and multiplier-rectifier circuits comprising a number of cascade stages.
FIGS. 1A and 1B illustrate two known high voltage multiplier circuits. In each of these circuits an AC input signal on a line 20 is coupled to a multiplier-rectifier circuit, 22, 24, respectively, which provides an output signal that is proportional to the input AC voltage amplitude on the line 20 and the number of multiplying stages. To reduce size and cost, the multiplier-rectifier circuits 22, 24 typically include many stages and to use capacitors that have the lowest possible capacitance. However, as known, distributed stray shunt capacitance Cs associated with the multiplier-rectifier portion of the power supply limits the performance of multiplier circuits, especially for multiplier circuits employing many stages and low values of coupling capacitance, Cc. Alternating currents flowing in the stray shunt capacitance, Cs, reduce the output voltage of stages furthest from the AC drive circuit (i.e., from the AC input signal on the line 20).
The undesirable effects of stray shunt capacitance Cs can be partially overcome by installing a loading inductor, LT, on the last stage as shown in FIGS. 2A and 2B. Additional loading inductors installed at intermediate locations along the multiplier circuit can further reduce the undesirable effects of stray capacitance. The performance benefits of using loading inductors are disclosed in the publication by E. Everhart, P. Lorain, entitled The Cockcroft-Walton Voltage Multiplying Circuit, published in the Review of Scientific Instruments, vol. 24, no. 3, p.221-226, (March 1953). The performance benefits include that the voltage distribution from stage-to-stage can be made substantially more uniform, and the stages furthest from the AC power source can contribute equally or even more than stages close to the AC source. However, the voltage distribution from stage-to-stage becomes dependent on the operating frequency of the AC drive circuit. As disclosed in the above identified publication by Everhart et al., an optimum voltage distribution is defined as one in which the voltage of the first and last multiplier stages, V1 and VN respectively, have equal amplitudes. This optimum distribution is obtained when the AC power source is operated at an optimum frequency, ωopt, which depends on the stray capacitance, the coupling capacitance and the loading inductance.
The use of the loading inductors also causes the input impedance of the multiplier-rectifier circuit, Zm, to become strongly dependent on frequency. With the addition of loading inductor LT, the multiplier impedance, Zm, exhibits resonant behavior. Below the resonance frequency value the reactive component of multiplier impedance Zm is inductive, and above resonance frequency value the reactive component of Zm is capacitive. The optimum voltage distribution as defined above is obtained at a frequency value above the resonant frequency value, and therefore multiplier impedance Zm has a capacitive reactance when ω=ωopt. At resonance, the ratio of the top stage voltage to first stage voltage, VN/V1, achieves its maximum value, and VN/V1>1. As the drive frequency is increased above resonance, VN/V1, decreases monotonically and is equal to unity at ω=ωopt. Therefore, to achieve good uniformity it is desirable to operate the multiplier-rectifier circuit in the frequency range, ωres<ω<ωopt, which causes the multiplier impedance Zm to have a capacitive reactance.
The interface transformer between the AC drive and the multiplier-rectifier circuit further increases the capacitive load presented to the AC drive circuitry. Interwinding capacitance associated with the secondary winding contributes additional shunt capacitance seen by the AC drive circuit. The winding capacitance appears in parallel with the input terminals of the multiplier-rectifier circuit.
It is common practice in the design of high voltage power supplies to achieve efficient coupling by requiring that the AC drive circuit couple to a resonantly tuned circuit, in the case of the multiplier-rectifier circuit described herein, this has been accomplished by incorporating an additional parallel inductor, LIN, at the input terminals of the multiplier-rectifier circuit, or an equivalent inductor in parallel with the primary winding of the transformer, as indicated in FIGS. 3A-3B, respectively. The inductor value is chosen so that the resonant frequency produced by the inductor and the equivalent capacitance, CEQ, of the multiplier-rectifier circuit, ω=1/√{square root over (LINCEQ)} is equal to the desired operating frequency, for example ω=ωopt. This approach has drawbacks, especially for commercial applications where ease of servicing and maintenance is important. The system is highly tuned requiring careful adjustment of the driver frequency to the resonant frequency. In addition, a frequency shift of the AC drive or the resonance of the multiplier-rectifier circuit can result from thermal and mechanical effects. Components and subsystems are difficult to replace without retuning of the power supply system. In addition, the input inductor, LIN, is often a source of power loss because of the large circulating currents.
Therefore, there is a need for a high voltage power supply system that substantially overcomes the disadvantages of a resonantly tuned AC-drive circuit combined with a multiplier-rectifier circuit, to provide an AC drive circuit that couples power to a load that usually includes a substantial capacitive component.